1. Field of the Invention
The present invention relates to a method of measuring an eccentricity of an aspherical lens, etc., and a measuring apparatus for determining an axis of an aspherical surface.
2. Related Background Art
Known conventional art measuring apparatuses for measuring an eccentricity of an one-sided aspherical surface are disclosed in Japanese Patent Laid-Open Application Nos. 3-115944, 5-196540 and 7-128188. Each of those apparatuses is based on a method in which a spherical surface of a lens is set as an abutting measurement reference surface, a spherical center of the spherical surface is made coincident with an axis of a separate rotary holder, thereafter a rotation thereof is made, deviation quantities in a thrust direction, a virtual spherical center direction and an outside diametrical direction are measured, and the eccentricity is estimated from those deviation quantities.
The above measuring method has, however, an error in principle. At first, the error is derived from the fact that the conventional art method utilizes a spherical aligning property of the opposite spherical surface in terms of the principle. Namely, an aligning reference when measuring the aspherical surface is set on the spherical surface, and hence the conventional art method has the following factors for causing the error.
For instance, Japanese Patent Laid-Open Application No. 5-196540 has such a construction that the lens is aligned along the opposite spherical surface (i.e., on the basis of the spherical center of the opposite spherical surface) so as not to move a chart image projected during the rotation of the lens, and an eccentric quantity is measured by detecting how much the spherical surface is displaced in an optical-axis direction parallel with a displacement meter when rotated by 180 degrees while following after the surface in that way. In a case such as treating an eccentric lens as shown in FIG. 1, wherein a spherical center Or2 of a spherical surface S2 does not exist on an aspherical axis A1 of an aspherical surface S1, however, even when rotated by 180 degrees about a pseudo optical axis A2 connecting a paraxial curvature center Oa1 of the aspherical surface S1 to a spherical center Or2 of the spherical surface S2, it never happens that the aspherical surface takes absolutely the same surface configuration disposition as the surface configuration before the rotation. That is, a deviation between the aspherical axis A1 and the spherical center Or2 can not be corrected by the rotation according to the spherical surface, and therefore the surface configuration disposition on the aspherical surface when rotated by 180 degrees is not absolutely coincident with the surface configuration disposition before the rotation. Hence, even when following after the surface so as not to move the chart image, the surface configuration disposition on the aspherical surface when rotated by 180 degrees can be just approximately closest to the surface configuration disposition before being rotated. Speaking of the control performed so as not to move the chart image, this does not mean that the control can be done strictly with no movement. There resultantly remains an error.
Further, in the Japanese Patent Laid-Open Application Nos. 7-128188 and 3-115944, the same error is caused for the reason of setting the pseudo optical axis A2 as a measuring basis, and, therefore, the precise measurement can not be performed in terms of principle. The following is a description of this problem with reference to FIG. 1.
Even if the aspherical axis A1 is aligned with the axis A2, defined as a rotary axis, by the conventional art method, it is impossible to correct the deviation between the spherical center Or2 and the aspherical axis A1 simply by the rotation according to the spherical surface as described above with respect to a tested body having an eccentricity. Therefore, there must invariably remain an angular deviation of an angle .theta.1 between the aspherical axis A1 and the rotary axis A2 passing through the spherical center Or2. Accordingly, the aspherical axis A1 might be tilted when setting A2 as a rotary axis. Hence, it follows that an actual measuring portion is not an annular zone of a curvature Ra1 and a curvature center Oa1 on the paraxial side but an annular zone of a curvature Ra2 and a curvature center Or2. The curvature of the annular zone is changed on the aspherical surface, and hence it follows that an apparent deviation angle .theta.2 attributed to a curvature difference of the annular zone of the measuring portion is formed as an error, excluding a deviation angle of an eccentric angle .theta.1 defined by the conventional art measuring method in that case. As far as the lens adjusting according to the spherical surface S2 is performed, the deviation described above can not be completely prevented in principle. Since A2 is set as the rotary axis, it is impossible to distinguish a deviation caused by the eccentricity from a deviation by variations in the curvature even when detecting peripheral deviations. The distinction becomes harder as an aspherical degree gets larger and as the eccentricity becomes larger. Hence, the rotary axis can not be approximately coincident with the aspherical axis in the adjustment based on this method.
In addition to the error factors described above, the measurement depending on the lens adjustment while displacing the spherical surface on the opposite side as a mechanical contact basis, contains a good number of error factors directly related to an accuracy for measuring the eccentricity, such as floating due to permeation of minute dusts, as well as a working precision of a spherical surface receiving portion of a lens holder.
Further, the biggest defect inherent in the conventional art methods is that both-sided aspherical surfaces can not be measured.